Mutations in deoB and deoC alter an extracellular signaling pathway required for activation of the gab operon in Escherichia coli

In Escherichia coli , a lacZ fusion to the gabT gene is activated by the accumulation of two self‐produced extracellular signals, indole and a second unidentified signal (signal‐2). Extracellular indole contributes approximately 25% of this activation and signal‐2 is responsible for the majority of activation. Using an E. coli strain unable to produce indole and containing a gabT::lacZ fusion, a genetic approach was used to search for genes involved in the production of signal‐2. A spontaneous E. coli mutant, MJ1, exhibited significantly less signal‐2 activity based on the ability of spent culture supernatants from this mutant to activate the gabT::lacZ fusion. Genetic analysis of MJ1 revealed that it contained two mutations, one in thyA and a second unknown mutation, designated spl1 ( s ignal p roduction l ocus) that led to loss of signal‐2 production. The spl1 second‐site mutation arises at high frequency in a thyA − background because it suppresses the loss of viability. This study demonstrates that mutations in deoB and deoC were capable of suppressing the loss of viability in thyA mutants and concomitantly resulted in loss of signal‐2 activity in conditioned medium. Interestingly, both deoB and deoC mutations in an otherwise wild‐type background resulted in higher levels of gabT::lacZ expression in cells at low density. It is hypothesized that deoB and deoC mutations result in an enhanced rate of signal‐2 uptake and thus deplete signal‐2 from the external medium.